A power electronic circuit sometimes introduces a reactive power causing a phase shift of the power supply current and voltage. Moreover, one or more conversion stages present in said electronic circuit, for example a stage for rectifying the voltage, cause deformations of the input current, consequently degrading the power factor. The power electronic circuit therefore requires, at its input, the addition of a correction circuit in order to increase the power factor.
On the one hand, the correction circuit, indicated by the acronym PFC in the rest of the description with reference to the expression “Power Factor Correction”, must put the current and the voltage back into phase. On the other hand, the PFC circuit must limit the harmonic distortions of the input current. Therefore, a PFC circuit must at least meet two constraints simultaneously: obtain a high power factor and a good quality of the induced-current harmonic distortion. The new standards, notably concerning purity in current shape are increasingly strict, as shown, for example, by the chapters relating to conducted emissions of the MIL-STD-461E standard of the American Defense Department.
To meet the aforementioned strict constraints with applications powered by a three-phase current, it is natural to juxtapose three PFC stages, one for each phase of the electric current. However, although this solution makes it possible to achieve good performance, both in terms of harmonic distortion and in terms of power factor, it culminates in a complex architecture, notably because of the balancing difficulties between the three PFC stages. Moreover, the resultant circuit is bulky because of the gearing-down of the components to be used.
An alternative solution using the principle of the PFC circuits of the “boost” type is shown in FIG. 1. It is a conventional correction circuit 100 for a three-phase power supply comprising a first filtering unit 101 dedicated to the low frequencies, a hexaphase rectifying bridge 102, a second filtering unit 103 dedicated to the high frequencies, and a voltage step-up stage 104, which comprises an inductor 105, a controlled switch 106, and a freewheel diode 107 powering a reservoir capacitor C. The value of the inductor 105 is chosen to be sufficiently large for the circuit 100 to operate in continuous mode. The capacitor C is a reserve of energy making it possible to power a user circuit, modeled in FIG. 1 by a load 110. This conventional correction circuit 100 makes it possible, without having recourse to three PFC stages, to significantly increase the power factor of the circuit. However, since the value of the inductor 105 has to be high to obtain an acceptable smoothing of the current, the inductive component chosen to fulfill this role is often very bulky. Moreover, in practice, the architecture of this circuit shows its limits in quality of the harmonic distortions; it does not make it possible to satisfy the requirements of the strictest standards.
Other solutions have been proposed, notably a circuit shown in the patent referenced U.S. Pat. No. 6,984,964 by the applicant Delta Electronics Inc. This circuit, designed for a three-phase power supply, makes it possible to obtain low levels of harmonic distortion while maintaining a high power factor. However, this circuit is particularly costly, because it requires the use of a Digital Signal Processor or DSP, and a complex programmable circuit or CPLD (“Complex Programmable Logic Device”), in order to control the backflows of current toward the input of the circuit notably when the neutral of the three-phase network is not connected to the circuit. Moreover, it is necessary to have 3 distinct PFC functions, one per phase in order to perform the “PFC” function making it possible to obtain all at the same time a power factor close to the unit combined with a low input-current harmonic distortion, for example in order to satisfy the requirement of the CE101 test of the MIL-STD-461E standard.